1. Field of the Invention
The present invention relates generally to a picture image forming system and a picture image forming method.
2. Related Background Art
Color picture image forming systems mainly use tandem systems for sequentially superposing and transferring images to obtain a color picture image by means of a plurality of picture image forming parts. Among such tandem systems, primarily used is a xe2x80x9ctrain-of-fourxe2x80x9d tandem system for superposing four colors (yellow, magenta, cyanogen and black).
Color picture image forming systems which utilize tandem systems offer the advantage of high printing speed. However, it is difficult to align the picture images in the respective colors, as a result, the positions of the aligned picture images in the colors are sometimes shifted to fade or blur characters and images, and the picture quality of the obtained color picture image is deteriorated. Therefore, in order to prevent the aligned picture images in the colors from being shifted, there is generally known a method for forming a test pattern, which is used for correcting the deviation in position of picture images formed by a plurality of picture image forming parts, on a conveying belt to detect the test pattern by a test pattern position detecting sensor to correct the deviation in position. According to this method, it is possible to precisely correct the deviation in position if the test pattern is normally formed.
However, in this method, there is a problem in that there are some cases where the width of the test pattern formed on the conveying belt is not normal due to environmental variation, deterioration with age and deterioration in consumable supplies when a picture quality maintaining control (a control for printing at an appropriate density) is not carried out, so that it is not possible to sufficiently precisely correct the deviation in position. Examples for explaining this problem are shown in FIGS. 7(a) through 7(c). In FIGS. 7(a) through 7(c), reference number 7 denotes a conveyor belt, and 9 denotes a toner for a test pattern. FIG. 7(a) shows an example where the width of the pattern is too narrow, FIG. 7(b) shows an example where the width of the pattern is too wide, and FIG. 7(c) shows an example where the width of the pattern is normal. For example, in general, the toner is difficult to adhere to the conveyor belt when conditions of low temperature and humidity exist as shown in FIG. 7(a), and excessive toner adheres to the conveyor belt under conditions of high temperature and humidity as shown in FIG. 7(b). If the test pattern is thus not normal, it is not possible to precisely detect the test pattern for each color, so that it is not possible to normally correct the deviation in position.
This will be described in more detail below. FIG. 8 shows an example of a detection signal indicative of the width of a test pattern. The test pattern position detecting sensor is designed to optically detect the presence of the pattern to output an analog signal indicative of the detected result. This analog signal is converted into a binary level digital signal based on an appropriate threshold as shown in FIG. 8. In FIG. 8, the width of the test pattern is detected by digital signals of H and L levels so that the test pattern exists at L level and no test pattern exists at H level. In FIG. 8, two test patterns having a pattern width of w are detected. For example, if the conditions for forming the picture image of the test pattern are varied due to the variation in environment, such as the conditions of low temperature and humidity, it is not possible to recognize the test pattern at a certain boundary value. Therefore, the width w of the test pattern is erroneously detected, so that it is not possible to normally correct the deviation in position.
As a method for solving this problem, Japanese Patent Laid-Open No. 10-260567 discloses a method for changing the conditions for forming the picture image of a test pattern in accordance with a value which is outputted from a density detecting sensor provided for detecting the density of the test pattern. This method intends to improve the precision of the correction of the deviation in position by maintaining the fact that the density of the test pattern detected by the test pattern position detecting sensor is always constant. However, this method can not sufficiently improve the precision of the correction of the deviation in position. That is, although this method can sufficiently improve the precision of the correction of the deviation in position if it can be assumed that the width of the test pattern is appropriate when the density of the test pattern is appropriate, the density and width of the pattern are, in fact, not always be coincident with each other, so that it is not possible to sufficiently improve the precision of the correction of the deviation in position.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a picture image forming system capable of obtaining a high picture quality by precisely correcting the deviation in position of picture images formed by a plurality of picture image forming parts.
In order to accomplish the aforementioned and other objects, according to one aspect of the present invention, there is provided a picture image forming system comprising: a plurality of picture image forming parts for forming picture images in accordance with a picture signals; test pattern forming means for causing the plurality of picture image forming parts to form test patterns which are used for correcting the deviations in position of a picture image formed by the plurality of picture image forming parts; detecting means for detecting the widths of the test patterns formed by the plurality of picture image forming parts; and changing means for comparing the widths of the test patterns read by the detecting means, with a predetermined reference widths to change the width of the test pattern on the basis of the compared results.
According to another aspect of the present invention, there is provided a picture image forming method for forming a picture image by superposing picture images on a recording medium using a plurality of picture image forming parts for forming the picture images in accordance with picture signals, the method comprising the steps of :causing the plurality of picture image forming parts to form test patterns which are used for correcting the deviation in position of a corresponding one of the picture images formed by the plurality of picture image forming parts; detecting the widths of the test patterns formed by the plurality of picture image forming parts; determining each the detected width of the test pattern within a predetermined reference width range; and correcting the deviations in position of the corresponding the picture images, which have been formed by the plurality of picture image forming parts, on the detected results for the test patterns when the widths of the test patterns are within the predetermined reference width ranges.